Browsing Survey & Cruise Reports by Author "Dransfeld, Leonie"
Celtic Sea Herring Acoustic Survey Cruise Report 2005O'Donnell, Ciaran; Doonan, Ian; Johnston, Graham; Lynch, Deirdre; Dransfeld, Leonie; Wall, David (Marine Institute, 2005)In the Celtic Sea and ICES Division VIIj, to the south and southwest of Ireland, herring are an important commercial species to the pelagic fleet. The local fleet is composed of dry hold polyvalent vessels and a number of purpose built RSW (Refrigerated seawater) vessels. This stock is composed of both autumn and winter spawning components. The commercial fishery has historically taken place within 6nmi (nautical miles) of the coast and focused on aggregated schools within the spawning cycle. In recent years the RSW fleet has actively targeted offshore summer feeding grounds in the south Celtic Sea. In division VIIj, the fishery traditionally begins in early October and is concentrated within several miles of the shore bays and inlets. The division VIIaS fishery peaks towards the year end in December, but may be active from mid October depending on location. In division VIIg, along the south coast herring are targeted from October to January at a number of known spawning sites and surrounding areas. Overall, the protracted spawning period of the two components extends from October through to January, with annual variation of up to 3 weeks. Spawning occurs in successive waves in a number of well known locations including large scale grounds and small discreet spawning beds. The stock structure and discrimination of herring in this area is not fully understood. It is known that fish in the eastern Celtic Sea recruit from nursery areas in the Irish Sea and tagging studies have shown linkages between these areas also. For the purpose of stock assessment and management these areas have been combined since 1982. For a period in the 1980’s, egg and larval surveys were conducted for herring in this area. However, since 1989, acoustic surveys have been carried out, and currently are the only tuning indices available. In the Celtic Sea and VIIj, herring acoustic surveys have been carried out since 1989, and the current survey is the 16th in the series. The autumn 2005 survey is the most comprehensive survey carried out in the current time series. The geographical confines of the annual 21 day survey were expanded to include areas to the south of the main winter spawning grounds in an effort to identify the whereabouts of winter spawning fish before the annual inshore spawning migration. Spatial resolution of acoustic transects were increased over the entire south coast survey areas. The acoustic component of the survey was complimented by a continuation of the detailed hydrographic work carried out in the Celtic Sea in 2004. In addition a second inshore survey was carried out along the southern coast using a chartered commercial fishing vessel (FV Regina Ponti). This second survey focused on area between 0.5nmi and 10nmi offshore and covered the main autumn and winter spawning areas along the south coast.